1. Field of the Invention
The present invention relates to growing crystals, such as silicon carbide (SiC) crystals, with a low content of unintentional nitrogen impurity.
2. Description of Related Art
Wafers of semi-insulating silicon carbide of 4H and 6H polytypes serve as lattice-matched substrates to grow epitaxial layers of SiC and AlGaN, which are used for the fabrication of SiC- and AlGaN-based microwave devices. In order for the device to work efficiently, the substrate must be semi-insulating, that is, fully compensated electronically. In practical terms, this means that the resistivity of the SiC substrate must be higher than 105 Ohm·cm, as measured at room temperature and under normal room light.
SiC crystals contain impurities and point defects, each with characteristic electronic levels in the bandgap. The levels that lay close to the edges of the bandgap are called “shallow”. In 4H and 6H SiC, nitrogen is a shallow donor. The levels positioned closer to the midgap are called “deep”. Vanadium and certain point defects produce deep levels in silicon carbide.
The net shallow impurity concentration in a semiconductor is defined as the absolute value of ND-NA, where ND and NA are the concentrations of shallow donors and acceptors, respectively. A semiconductor is fully compensated when the concentration of deep levels is higher than the net shallow impurity concentration. Doping with vanadium and introduction of deep point defects has been used for electronic compensation of silicon carbide.
Unintentional nitrogen donors can be present in SiC crystals at levels as high as 2·1017 cm−3. In order to achieve reliable compensation, the concentration of deep levels should be higher. In general, introduction of deep levels in high concentrations is technologically difficult and can cause stress and generation of defects. A better approach to full compensation would be through a reduced presence of unintentional shallow impurities, first of all, nitrogen.
Physical Vapor Transport (PVT) is the most common sublimation technique used for SiC crystal growth. A schematic diagram of conventional PVT is shown in FIG. 1. A polycrystalline SiC source 2 is loaded at the bottom of a graphite crucible 1, and a SiC seed 4, desirably a single crystal SiC seed 4, is disposed at the top on the lid. The crucible 1 is heated to a growth temperature, generally, between 2000° C. and 2400° C., where source 2 vaporizes and fills the crucible with volatile molecular species of Si2C, SiC2 and Si. During growth, the source 2 temperature is higher than the seed 4 temperature. This temperature difference forces the vapors to migrate and precipitate on seed 4, forming an SiC crystal 3, desirably an SiC single crystal. In order to control the growth rate and ensure high crystal quality, PVT growth is carried out under a small pressure of inert gas, generally between several and 200 Torr. Conventional PVT is a “closed” process in which the Si-bearing vapors do not leave the container, except unintentional losses that can occur through the joint between the crucible body and lid.
SiC crystals have also been grown using an “open” process. In the open process, a deliberate gas flow is established between the crucible interior and exterior. Examples include High Temperature Chemical Vapor Deposition (HTCVD), Halide Chemical Vapor Deposition (HCVD) and some PVT modifications. A generalized diagram of an open SiC crystal growth process is shown in FIG. 2. A gas mixture 11 that may contain Si and C precursors, dopants and/or other reactants enters crucible 10 through inlet 12. Inside crucible 10, the reactants undergo chemical transformations in a reaction zone 14. The reaction products blend with the vapors originating from the polycrystalline SiC source 13 and move toward a SiC seed 17, where they precipitate and form crystal 15. Gaseous products escape through outlet(s) 16 in the lid of crucible 10.
Graphite is widely utilized in SiC crystal growth as a material for crucibles and other parts. Due to the chemical nature and porosity of graphite, it is capable of adsorbing large quantities of air, for instance, up to 100 cc of air per gram of graphite. Upon heating to high temperatures, graphite releases the adsorbed gas, thus becoming one of the principal sources of nitrogen contamination in SiC crystal growth. The present inventors have discovered that another source of nitrogen contamination is N2 from the growth station chamber filtering into the crucible interior through the permeable graphite wall of the crucible.
Migration of a gas through a porous body can be described in terms of permeation or diffusion, which are physically equivalent. While permeability is defined by the volume of gas flowing across a porous wall of unit thickness under unit pressure drop, the diffusivity is defined by the number of molecules moving under unit concentration drop. Gas migration in graphite involves steps of gas adsorption on the pore surface, desorption, surface diffusion and bulk diffusion. Permeability of graphite depends on the nature of the gas and properties of graphite: low-density and open-porosity graphite is more permeable than high-density and closed-porosity graphite. Gases with larger molecules often diffuse faster than those with smaller molecules due to the more extensive trapping of small molecules in the micro-pores of graphite. Extensive trapping and long lifetime in the trapped state slows down diffusion of elements with high chemical affinity for carbon.
Nitrogen contamination in SiC crystal growth can be reduced to some extent using common and well-known conventional techniques. One such technique is evacuation and purge of the chamber with pure inert gas. This technique, however, fails to achieve complete removal of nitrogen from graphite bulk. Another conventional method is based on the use of getters such as titanium. However, application of getters in the conditions of PVT growth is difficult and can lead to severe contamination of the crystal.
A better purity with respect to nitrogen can be achieved in open growth processes, such as HTCVD and HCVD, where the nitrogen concentration in the grown crystals can be reduced to below 1016 cm−3. This can be attributed to the “dilution” effect of the flowing gas and/or to the presence of hydrogen in the growth atmosphere. However, these open growth processes failed to demonstrate high crystal yields, mostly due to the very high losses of silicon-bearing vapors escaping through the crucible outlet(s).